The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The present disclosure is related in general to fluid compositions and methods for servicing subterranean wells. Particularly, the disclosure relates to the use of fluids containing fibers to support another wellbore-servicing fluid above, and to prevent their commingling. Such fluids containing fibers are also referred to as “fiber-laden fluids.”
Well-servicing operations frequently involve the pumping and placement of process fluids in the wellbore. Such process fluids may include (but not be limited to) drilling fluids, spacer fluids, chemical washes, cement slurries, acidizing fluids, fracturing fluids, formation-consolidation fluids and gravel-pack fluids.
In many cases, different process fluids are pumped through tubulars in a sequence. Perhaps the most common example is primary well cementing, during which the fluid sequence may involve drilling fluid, followed by a spacer fluid, and then followed by one or more cement-slurry formulations.
Similar situations may occur during remedial cementing. Remedial cementing is a general term to describe operations that employ cementitious fluids to cure a variety of well problems. Such problems may occur at any time during the life of a well, from well construction to well stimulation, production and/or abandonment. Plug cementing is a subset of remedial cementing, during which a discrete volume of cement slurry is placed in a wellbore and allowed to set. There may or may not be casing present in the well. Plug cementing is most commonly performed during the following scenarios: sealing lost-circulation zones, sidetracking around a fish (lost object or other debris in the hole), initiating directional drilling, sealing a depleted zone, protecting a low-pressure zone during a workover treatment and sealing wells for abandonment.
In most cases, cement plugs are placed at a distance above the bottom of the well, which may be filled with drilling fluid or completion fluid. In many cases, the density of the cement slurry will be higher than that of the drilling or completion fluid. If no precautions are taken, the slurry will likely fall to the bottom of the hole.
Several techniques have been disclosed in the art to prevent or minimize downward movement of the cement plug after placement. The methods fall into two principal categories: hydrodynamic or mechanical.
Hydrodynamic techniques may involve adjusting the rheological properties of the cement plug. For example, thixotropic cements are frequently used. In practical terms, thixotropic slurries are fluid during mixing and displacement but rapidly form a rigid, self-supporting gel structure when pumping ceases. Thus, after placement, the slurry gel structure helps prevent commingling with the fluid below.
Another hydrodynamic technique is to place a viscous pill between the cement plug and the drilling or completion fluid, thereby providing support for the cement slurry. For example, a viscous pill may comprise a crosslinked-polymer fluid which may also contain lost-circulation materials (LCMs). The pills are usually rubbery, ductile or spongy, and have little compressive strength after setting. Other examples include gelled sodium-silicate solutions and gelled hydrocarbons. The crosslinking or thickening may be activated by time, temperature or shearing through a drill bit. In most cases, the fluid composition and rheological properties must be custom designed for a particular application. A detailed description of these fluids is presented in the following publication: Daccord G, Craster B, Ladva H and Jones T G J: “Cement-Formation Interactions,” in Nelson E B and Guillot D (eds.): Well Cementing 2nd Edition, Schlumberger, Houston (2006) 191-232.
Mechanical methods to support a cement plug include devices such as inflatable packers. When the tool reaches the target location, the inflatable packer expands to provide a mechanical support. An umbrella-shaped tool may be deployed below the cement plug to help support said cement plug. Also, a diaphragm bow, pumped like a cement plug through drillpipe, expands when it exits and acts as a hydraulic barrier. These devices effectively provide support for a cement plug; however, special hardware at the wellhead is required to deploy them. In addition, working with these devices is time consuming. A description of these devices is presented in the following publication: Daccord G, Guillot D and James S: “Remedial Cementing,” in Nelson E B and Guillot D (eds.): Well Cementing 2nd Edition, Schlumberger, Houston (2006) 503-547.
Despite the valuable contributions from the art, it remains desirable to provide means to support a cement plug without the need to tailor a fluid composition or rheological properties, and without the need to provide special hardware at the wellsite.